Performance of wireless communication systems is directly related to signal strength statistics of received signals. Third generation wireless communication systems utilize transmit diversity techniques for downlink transmissions (i.e., communication link from a base station to a mobile-station) in order to improve received signal strength statistics and, thus, performance. Two such transmit diversity techniques are space time spreading (STS) and phase sweep transmit diversity (PSTD).
FIG. 1 depicts a wireless communication system 10 employing STS. Wireless communication system 10 comprises at least one base station 12 having two antenna elements 14-1 and 14-2, wherein antenna elements 14-1 and 14-2 are spaced far apart for achieving transmit diversity. Base station 12 receives a signal S for transmitting to mobile-station 16. Signal S is alternately divided into signals se and so, wherein signal se comprises even data bits and signal so comprises odd data bits. Signals se and so are processed to produce signals S14-1 and S14-2. Specifically, se is multiplied with Walsh code w1 to produce signal sew1; a conjugate of signal so is multiplied with Walsh code w2 to produce signal so*w2; signal so is multiplied with Walsh code w1 to produce sow1; and a conjugate of signal se is multiplied with Walsh code w2 to produce se*w2. Signal sew1 is added to signal so*w2 to produce signal S14-1 (i.e., S14-1=sew1+so*w2) and signal se*w2 is subtracted from signal sow1 to produce signal S14-2 (i.e., S14-2=sow1−se*w2). Signals S14-1 and S14-2 are transmitted at substantially equal or identical power levels over antenna elements 14-1 and 14-2, respectively. For purposes of this application, power levels are “substantially equal” or “identical” when the power levels are within 1% of each other.
Mobile-station 16 receives signal R comprising γ1(S14-2)+γ2(S14-2), wherein γ1 and γ2 are distortion factor coefficients associated with the transmission of signals S14-1 and S14-2 from antenna elements 14-1 and 14-2 to mobile-station 16, respectively. Distortion factor coefficients γ1 and γ2 can be estimated using pilot signals, as is well-known in the art. Mobile-station 16 decodes signal R with Walsh codes w1 and w2 to respectively produce outputs:W1=γ1se+γ2so  equation 1W2=γ1so*−γ2se*  equation 1aUsing the following equations, estimates of signals se and so, i.e., ŝe and ŝo, may be obtained:ŝe=γ1*W1−γ2W2*=se(|γ1|2+|γ2|2)+noise  equation 2ŝo=γ2*W1+γ1W2*=so(|γ1|2+|γ2|2)+noise′  equation 2a
However, STS is a transmit diversity technique that is not backward compatible from the perspective of the mobile-station. That is, mobile-station 16 is required to have the necessary hardware and/or software to decode signal R. Mobile-stations without such hardware and/or software, such as pre-third generation mobile-stations, would be incapable of decoding signal R.
By contrast, phase sweep transmit diversity (PSTD) is backward compatible from the perspective of the mobile-station. FIG. 2 depicts a wireless communication system 20 employing PSTD. Wireless communication system 20 comprises at least one base station 22 having two antenna elements 24-1 and 24-2, wherein antenna elements 24-1 and 24-2 are spaced far apart for achieving transmit diversity. Base station 22 receives a signal S for transmitting to mobile-station 26. Signal S is evenly power split into signals s1 and s2 and processed to produce signals S24-1 and S24-2, where s1=s2. Specifically, signal s1 is multiplied by Walsh code wk to produce S24-1=s1wk, where k represents a particular user or mobile-station. Signal s2 is multiplied by Walsh code wk and a phase sweep frequency signal ej2πfst to produce S24-2, i.e., S24-2=s2wkej2πfst=s1wkej2πfst=S24-1ej2πfst, where fs is a phase sweep frequency and t is time. Signals S24-1 and S24-2 are transmitted at substantially equal power levels over antenna elements 24-1 and 24-2, respectively. Note that the phase sweep signal ej2πfst is being represented in complex baseband notation, i.e., ej2πfst=cos(2πfst)+j sin(2πfst). It should be understood that the phase sweep signal may also be applied at an intermediate frequency or a radio frequency.
Mobile-station 26 receives signal R comprising γ1S24-1+γ2S24-2. Simplifying the equation for R results inR=γ1S24-1+γ2S24-1ej2πfst  equation 3R=S24-1{γ1+γ2ej2πfst}  equation 3aR=S24-1γeq  equation 3bwhere γeq is an equivalent channel seen by mobile-station 26. Distortion factor coefficient γeq can be estimated using pilot signals and used, along with equation 3b, to obtain estimates of signal s1 and/or s2.
In slow fading channel conditions, PSTD improves performance (relative to when no transmit diversity technique is used) by making the received signal strength statistics associated with a slow fading channel at the receiver look like those associated with a fast fading channel. However, PSTD causes the energy of the transmitted signals to be concentrated at some frequency between the carrier frequency and the phase sweep frequency. If the frequency at which the transmitted signals are concentrated is not within some frequency tolerance of a mobile-station or receiver to which the signals are intended, the mobile-station or receiver may not be able to or may have difficulty receiving or processing the signals which, in turn, may degrade performance. Accordingly, there exists a need for a transmit diversity technique that is backward compatible without degrading performance.